Expandable head rivet

ABSTRACT

An expandable head rivet for sealing applications and the like having improved inspectability after setting. The rivet has an axially extending shank, a first substantially cylindrical section and a second tapered section between the first section and the shank, all generally conforming to the hole and counterbores in the workpiece. The face of the rivet head has a raised central section having a substantially flat surface perpendicular to the axis of the rivet of a cross sectional area of at least a substantial percentage of the cross sectional area of the shank of the rivet. Upon setting of the rivet in the workpiece, the head end of the rivet is flattened and radially expanded to tightly engage the cylindrical counterbore. In the event that the cylindrical counterbore in the workpiece is longer than intended and longer than the corresponding section of the rivet head, the periphery of the endface of the set rivet will be disposed within the counterbore, thereby providing a visual indication that the counterbore was deeper or longer than expected, and also providing a visual indication of the possibility that the frustro-conical section of the rivet is not in tight engagement with the frustro-conical counterbore in the workpiece.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of solid rivets.

2. Prior Art

Solid rivets of various kinds are of course very well known in the priorart. Of particular importance to the present invention are solid rivetsof the type which are generally used for sealing applications such as,by way of example, may be used in the construction of aircraft wing fueltanks and the like. In such applications, the rivets are intended notonly to join two or more metallic sheets or members, but to also providea seal for the rivet hole so that there will be no fuel leak around therivet.

One such sealing rivet was developed for use in fuel sealingapplications for the Boeing Aircraft model 727 wing structure. Thatrivet, identified as the BACR15DG rivet, was intended for use with aworkpiece having a bore, a first substantially cylindrical counterboreand a second counterbore which is frustro-conical and tapers forwardlybetween the first counterbore and the bore. The rivet itself had a headincluding an endface, a first cylindrical section located forwardly ofthe endface and a second tapered section forwardly of the first sectionbetween the first section and the rivet shank, the tapered section ofthe rivet substantially matching the taper of the frustro-conicalcounterbore of the workpiece. The entire endface of the head end of therivet had a dome shape defined by a predetermined spherical radius of anamount dependent upon the size of the rivet. When the rivet was placedin position in the workpiece and set using a flat or substantially flattool against the head end of the rivet, the dome shape of the rivet wasflattened, substantially expanding the first section of the head intotight fuel-sealing engagement with the first counterbore in theworkpiece.

The characteristics of the BACR15DG rivet in comparison to thecounterbores in the workpiece before setting the rivet and thecharacteristics of the rivet after setting are best illustrated byconsidering the specific dimensions for a specific rivet of the BACR15DGseries. By way of example, for the 3/16ths diameter shank rivet, thecombined height of the conical section and the cylindrical section ofthe head range from 0.085 inches to 0.090 inches, with the correspondingdimensions for the hole specification ranging from 0.075 to 0.085inches. Thus the periphery of the head of the rivet prior to settingwould range from flush to 0.015 inches above the surface of theworkpiece. The diameter of the first section of the 3/16ths rivet rangesfrom 0.2700 to 0.2725 inches, with the diameter of the correspondingcounterbore in the workpiece ranging from 0.269 to 0.274 inches. Thusthe head of the 3/16ths rivet can range from having a 0.002 inch radialclearance to approximately a 0.0018 inch radial interference with theworkpiece before setting the rivet. As set, the specification for theunshaven head height for the 3/16ths inch BACR15DG rivet was 0.002 to0.010 inches. In aerodynamic applications, it is generally required thatthe head height of a set rivet be in the range of 0.002 inches to 0.005inches, the upper limit being determined by aerodynamic considerationsand the lower limit being determined as assuring that the face of thesetting tool "bottomed" on the head of the rivet, not on the workpiece.Given these dimensions and tolerances, it is therefore apparent thatstatistically most of the 3/16th BACR15DG rivets were probably eitherslip fit or a lose fit in the counterbore in the workpiece prior tosetting. Also, the dimensions of the heads of many of the set rivetswould satisfy the head protrusion limits for aerodynamic applications,though clearly many others would require shaving after setting to meetsuch requirements.

A very similar rivet is disclosed in U.S. Pat. No. 4,000,680 entitled"Expanding Head Riveting Method and System". In that patent a domed headrivet is disclosed wherein the spherical section of the dome extendsradially outward to a position short of the periphery of the first orcylindrical section of the head, curving into the otherwise flat endfaceof the head just outboard of the shank diameter. Also, the cylindricalsection of the head is of a diameter less than but about the same as thediameter of the first counterbore to always provide clearancetherebetween prior to setting. Subsequent patents to the same inventor,including U.S. Pat. Nos. 4,051,592, 4,086,839 and 4,156,666, related tothe foregoing patent, disclose a dome headed rivet having what isreferred to as a "ring dome" rather than a central dome, the ring domebeing aligned with the outer diameter of the rivet shank. Rivets of thislatter type have enjoyed substantial commercial success and are commonlyused in fuel sealing aircraft applications wherein the workpiece has acylindrical counterbore followed by the second frustro-conicalcounterbore joining the cylindrical counterbore with the bore throughthe workpiece.

The cylindrical head of the ring domed rivets expands radially onsetting of the rivet to tightly engage the cylindrical counterbore inthe workpiece when the counterbore is of proper depth, with the head ofthe set rivet generally being within the limits for aerodynamic usagewithout shaving of the head. At the same time the conical section of therivet is generally in firm engagement with the frustro-conical taperedsection of the hole in the workpiece, giving the desired strength andfatigue resistance for the set rivet. However sometimes the firstcounterbore (and thus the tapered section) of the hole in the workpieceis drilled too deeply. In such cases when a ring domed rivet isinstalled in such a deep counterbore, the periphery of the head remainssubstantially flush with the top of the workpiece and the head isflattened to at least somewhat radially expand the head so that theresulting installation has substantially the same appearance as adimensionally correct and properly installed ring domed rivet in adimensionally correct counterbored hole in the workpiece.

However in the case of the rivet set in the deep counterbore, the rivetbecomes forced upward off of the conical portion of the counterbore,leaving a small gap between the tapered portion of the counterbore andthe tapered section of the rivet head. This small gap allows the partsto move relative to each other under repetitive stress and vibration,resulting in a loosening of the head and the development of a fuel leaktherearound. Accordingly, with these prior art rivets the only way toassure a proper joint is to first assure that the counterbored holes inthe workpiece are within specification tolerances to start with byappropriate inspection thereof. The inability to inspect for thiscondition after the setting of the rivet is a substantial disadvantagehowever, as there is no opportunity to detect a lack of inspection ofthe holes or alternatively, an improper inspection thereof, after thering dome rivets have been set. Accordingly, one of the primary objectsof the present invention is to provide an expanding head rivet for fuelsealing and similar applications which can be inspected after thesetting of the rivet so as to reveal an excessively deep counterbore asopposed to a proper counterbore, and for that matter to also reveal ashallow counterbore, all in a rivet which when properly installed in aproper counterbore will not require shaving in aerodynamic applications.The same will also occur with the center domed rivets of U.S. Pat. No.4,000,680, as the dome is almost entirely within the shank diameter.

In addition to the foregoing prior art, also relevant for backgroundpurposes are U.S. Pat. Nos. 3,526,032, 3,747,467, 3,927,458, and4,004,484. Also relevant are the following foreign patents: RussianPatent No. 516,848, German Patent No. 707,103, Russian Patent No.409,016 and Russian Patent No. 517,709.

BRIEF SUMMARY OF THE INVENTION

An expandable head rivet for sealing applications and the like havingimproved inspectability after setting. The rivet has an axiallyextending shank, a first substantially cylindrical section and a secondtapered section between the first section and the shank, all generallyconforming to the hole and counterbores in the workpiece. The face ofthe rivet head has a raised central section having a substantially flatsurface perpendicular to the axis of the rivet of a cross sectional areaof at least a substantial percentage of the cross sectional area of theshank of the rivet. Upon setting of the rivet in the workpiece, the headend of the rivet is flattened and radially expanded to tightly engagethe cylindrical counterbore. In the event that the cylindricalcounterbore in the workpiece is longer than intended and longer than thecorresponding section of the rivet head, the periphery of the endface ofthe set rivet will be disposed within the counterbore, thereby providinga visual indication that the counterbore was deeper or longer thanexpected, and also providing a visual indication of the possibility thatthe frustro-conical section of the rivet is not in tight engagement withthe frustro-conical counterbore in the workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross section of a rivet and associated hole in aworkpiece in accordance with a preferred embodiment of the presentinvention.

FIG. 2 is a partial cross section similar to that of FIG. 1 showing asetting anvil positioned to engage the flat surface on the head end ofthe rivet.

FIG. 3 is a cross section similar to FIG. 2 illustrating the rivet afterthe same has been partially set.

FIG. 4 is a cross section similar to FIGS. 2 and 3 showing the rivet inthe fully set condition.

FIG. 5 is a cross section similar to FIG. 2 showing the rivet positionedin a slightly deep counterbore prior to setting.

FIG. 6 is a cross section similar to FIG. 5 showing the rivet in theslightly deep counterbore after setting.

FIG. 7 is a cross section similar to FIGS. 4 and 6 though showing therivet in a very deep counterbore after the same has been set.

DETAILED DESCRIPTION OF THE INVENTION

First referring to FIG. 1, the preferred embodiment of the presentinvention in the unset state and positioned within a proper counterborein a workpiece may be seen. In the description to follow, the majorelements of the rivet shall be referred to as the rivet shank 20, afirst generally cylindrical section 22 and a second frustro-conicalsection 24 joining the first section 22 and the shank 20 of the rivet.The head endface of the rivet, generally indicated by the numeral 26,extends to a position above the periphery 28 of the first section 22 ofthe rivet. The top endface 26 is defined primarily by two surfaceregions, namely a substantially flat region 30 centered on the axis 32of the rivet and located in a plane perpendicular thereto, and anannular region 34 therearound tapering from the central raised area 30to the periphery 28 of the head endface, in the preferred embodiment nota linear taper.

In general herein, references to the rivet head generally are referencesto the first section 22 and the material defining the top endface,though may include the second section 24, depending upon the nature andcontext of the reference. The hole in the workpiece into which the rivetis placed for setting will be referred to generally herein as the holeor the counterbored hole, generally being comprised of three sections,namely a through bore into which shank 20 fits, a first substantiallycylindrical counterbore 36 and a second frustro-conical counterboreregion 38 joining the first counterbore 36 with the bore 40 through theworkpiece. In general, references herein simply to the counterborenormally will refer to the combination of the first counterbore 36 andthe second counterbore 38. By way of example, the desired orspecification counterbore is a counterbore wherein the depth of thefirst counterbore 36 is within a predetermined dimensional range, theposition of the frustro-conical counterbore 38 of course depending uponthe actual length of the first counterbore 36. A deep counterbore, onthe other hand, generally indicates that the length or depth of thefirst counterbore 36 is greater than the desired dimensional limits,with the frustro-conical counterbore 38 similarly being positioneddeeper into the workpiece, specifically the upper member 42 of theworkpiece, than desired or allowed by specification. In general theangle of frustro-conical section 24 of the rivet generally matches theangle of the second counterbore 38 of the hole in the workpiece.

In general, it is desired to have the rivet slip into the hole in theworkpiece and the head of the rivet into the counterbored portionthereof without being forced. Accordingly, the diameter of the firstsection 22 of the rivet head should have an outer diameter which is lessthan but about the same as the diameter of the first counterbore 36.Normally this will provide a radial gap between the first section 22 andthe wall of the first counterbore 36 ranging from essentially 0 tosomething on the order of 0.002 inches, depending on such factors as therivet size and of course the specific design choices. The length of thefirst section 22 of the rivet head should be no less than the maximumdepth of the first counterbore 36 for a proper counterbored hole in theworkpiece. Thus whatever the maximum allowed depth is for the firstcounterbore 36 as per the specification therefor, the minimum length ofthe first section 22 should not be less than that maximum depth. Thus inthe limit, the periphery 28 of the head endface 26 of the rivet mayrange from initially being flush with the surface of the workpiece tobeing somewhat thereabove, typically having an upper limit on the orderof 0.004 inches for a 3/16ths inch diameter rivet. These numbers ofcourse are representative only, though indicative of suitable dimensionsand tolerances for the rivet and hole dimensions.

The flat raised central section 30 of the rivet head should have an areaof at least a substantial percentage of the cross sectional area of theshank of the rivet for reasons to be hereinafter described in greaterdetail. The surface 34 of the head endface connecting the periphery ofthe central raised area 30 with the periphery 28 of the first section ofthe rivet head will generally taper from the raised area 30 to theperiphery 28. In the preferred embodiment, the tapering surface 34 isnot a linear or conical taper, but rather is a surface whichapproximates a shape defined by a portion of a torus which is tangent atthe periphery 28 of the first section 22 to a plane perpendicular to therivet axis, and which portion of the torus curves therefrom to interceptthe periphery of region 30. This is illustrated in FIG. 1 wherein thecenter of the generating radius R for the curve of the cross section isspaced outward from the axis of the rivet by an amount equal to theradius of the first section 22 of the rivet. As shall be subsequentlyexplained in greater detail, when the rivet is set, the raised area 30is flattened, the material thereof being pushed downward generally intothe first section 22 of the head, expanding the same radiallysubstantially more than the clearance between the first section and thefirst counterbore, thereby providing a strong interference fittherebetween to create a strong fuel seal. In the preferred embodimentthe area of the central raised region 30 is approximately 70% of thecross sectional area of the shank 20 of the rivet. This amounts to adiameter for the central raised area of approximately 84% of the shankdiameter.

The importance of the parameters defining the head endface of the rivetare perhaps best illustrated by considering cross sections of the rivetin an unset state, a partially set state and in the fully set state incounterbores of various depths. Thus referring now to FIG. 2, a crosssection of a rivet in accordance with FIG. 1 in a counterbored hole in aworkpiece of proper depth and width and with anvil 44 of a setting toolin contact with the central region 30 of the rivet head may be seen. Ingeneral, rivet setting tools have an anvil such as anvil 44 whichcontacts the head end of the rivet, an opposite anvil (not shown) forcontacting the shank end of the rivet and a spring-loaded memberconcentric to the last named anvil and pressing against the bottom ofthe workpiece. In this manner the force on the head end of the rivetduring setting will be equal to the setting force on the shank end ofthe rivet plus the force of the spring-loaded member acting on theunderside of the workpiece, so long as the anvil 44 does not itselfcontact the top surface of the workpiece. The net result is that thereis a net force on the rivet itself forcing the rivet into thecounterbored hole in the workpiece during setting which is equal to theforce of the spring-loaded member against the underside of theworkpiece, though this spring-force is normally only a small percentageof the required setting force for the rivet.

Now referring to FIG. 3, as the force between the two anvils isincreased to initiate setting of the rivet, the first portion of therivet to yield in compression is the upward extending area 30 in thehead end of the rivet into the first cylindrical section 22 of therivet, forcing the same to expand radially to fill the initial gapbetween the central section 22 and the first counterbore 36 and to begina forceful tight fit therein, the raised central section 30 and theassociated part of the head thereunder being the first region to yieldbecause of the same having a cross sectional area which is less than thecross sectional area of the rivet shank. As this yielding in compressioncontinues however, the area of the head end of the rivet in contact withthe anvil 44 increases, so that once the area of the head in contactwith anvil 44 approximates the cross sectional area of the shank 20 ofthe rivet, the upset at the lower end of the rivet begins. Thisintermediate setting condition is illustrated in FIG. 3. As settingcontinues, the flattening and expansion of the head end of the rivetcontinues until setting is complete, as illustrated in FIG. 4. When thisstage is reached either the entire head or substantially the entire headhas been flattened and the further advance of anvil 44 stopped becauseof such flattening. In this condition the anvil has either just barelyreached the top surface of the workpiece or is still spaced somewhattherefrom by an amount which as previously stated typically will rangefrom 0 to 0.004 inches. Such an installation as is illustrated in FIGS.2 through 4 is the ideal installation, both the rivet and the hole beingdimensioned so that as long as both are within tolerance the head willbe expanded to provide a very tight fuel seal, the conical portion ofthe head will be in firm engagement with the conical portion of thecounterbore and the height of the rivet head above the workpiece willrange from flush to typically on the order of 0.004 inches, thereby notrequiring any after installation shaving in aerodynamic applications.

Now referring to FIG. 5, a diagram showing a rivet in accordance to thepresent invention in a slightly deep counterbore prior to setting may beseen. The phrase "slightly deep" means that the counterbore is somewhatdeeper than the maximum allowed by design as indicated by the fact thatthe periphery of the endface 28 before setting is lower than the topsurface of the workpiece 42. In this case, when the rivet is set asshown in FIG. 6, the raised central section 30 of the rivet will stillflatten sufficiently so as to radially expand the cylindrical section 22of the rivet to provide a fuel tight seal between the head and thecounterbore, the flattened head having an area in contact with anvil 44as large as or still slightly larger than the cross sectional area ofthe shank 20 of the rivet so that the tapered portion of the rivet willbe kept in firm engagement with the conical counterbore. The conditioncan be inspected after setting of the rivet however, by inspecting forthe characteristic positioning of the peripheral area 50 below thesurface 48 of the workpiece, and limits may readily be set either forthe maximum depth of the surface 50 below the workpiece surface and/orthe radial extent of the annular area 50 within which flattening duringsetting of the rivet has not occurred.

If on the other hand the counterbore in the workpiece is somewhat deeperyet, there will still be a flattening of the head end of the rivet sothat an area approximately equal to the cross sectional area of theshank results, though if the counterbore is sufficiently deep for theanvil 44 to bottom against the surface 48 of the workpiece before thatlimited flattening occurs, the force from the anvil on the other end ofthe rivet will cause the rivet to move upward as the head flattens,creating a void in region 52 just under the head. Also some radialexpansion of region 22 of the rivet head will occur, though that radialexpansion will be limited, not providing as tight a seal as desired.Further, the expansion tends to be concentrated near the top of section22 of the rivet, primarily because the lower part thereof was not reallyconfined during the setting procedure. Such a condition, readilyindicated by inspection after setting of the rivet by the size andradial extent of region 50a, is indicative of a riveted joint which willhave a reduced strength and a substantially reduced fatigue life beforeleaking because of the grossly reduced resistance of the joint parts tomove relative to each other under the effects of repetitive stressingand vibration. Thus the size of region 50 (FIG. 6) and 50a (FIG. 7)and/or the extent to which the periphery of the rivet head is depressedbelow the surface of the workpiece provides a readily visuallyperceivable and inspectable condition to determine the quality of theriveted joint after the rivet is set.

The foregoing is to be compared to the prior art rivets, particularlyeither the ring dome rivets hereinbefore referred to or the centrallydomed rivets such as disclosed in U.S. Pat. No. 4,000,680. In both casesthe area of the head of the rivet in contact with the setting anvilgenerally remains smaller than the cross sectional area of the shank sothat the head of the rivet during setting tends to be nearly fullyflattened before the shank end is substantially formed. In the case of adeep counterbore in the workpiece wherein the setting anvil at the topof the rivet head bottoms on the workpiece before the head is fullyflattened and expanded, the remaining flattening of the rivet headoccurs by way of the lower anvil forcing the entire rivet upward, againcreating a void under the rivet head equivalent to the void 52 of FIG.7. Because of the design of these heads, the same will still pretty muchentirely flatten, creating the visual appearance of a very goodinstallation, yet providing either a weak or nonexistent fuel seal andbeing susceptible to developing premature leaks and even a possiblestress or fatigue failure. It is the inspectability of the presentinvention after setting which primarily distinguishes the same fromother expanding head rivets for fuel sealing applications, particularlysuch prior art rivets as are intended to be used in aerodynamic as wellas other applications without requiring the shaving of a head or atleast selective shaving of the rivet heads after installation.

I claim:
 1. For combination with a workpiece having a bore, a firstcounterbore of a first length which is substantially cylindrical, and asecond counterbore which is frustro-conical and tapers forwardly betweenthe first counterbore and the bore:(a) a rivet having an axiallyextending shank receivable in the bore and defining an axis; (b) therivet having a head including an endface, a first section locatedforwardly of the endface, and a second and forwardly tapered sectionlocated forwardly of the first section; (c) the first section having aperiphery with an undeformed length which is about the same as but notless than the length of the first counterbore in the workpiece; (d) thefirst section having an undeformed outer surface of a diameter which isless than but about the same as the diameter of the first counterbore sothat the first section is radially outwardly deformable into tightperipheral engagement with the first counterbore; (e) said head endfacehaving a raised central section having a substantially flat surfaceperpendicular to the axis of the rivet, the ratio between the area ofsaid raised central section and the cross sectional area of said shankbeing constant for all rivet sizes and types, the ratio beingapproximately 70%, said head endface tapering from said raised centralsection to the periphery of said first section.
 2. The rivet of claim 1wherein said first section has a periphery with an undeformed lengthranging from the same as to 0.004 inches more than the length of thefirst counterbore in the workpiece.
 3. The rivet of claims 1 or 2wherein the taper from said raised central section to the periphery ofsaid first section approximates a shape defined by a portion of a toruswhich is tangent at the periphery of said first section to a planeperpendicular to the rivet axis and which curves away therefrom tointercept said substantially flat surface of said raised central sectionat the periphery thereof.
 4. In combination with a workpiece having abore, a first counterbore of a first length which is substantiallycylindrical, and a second counterbore which is frustro-conical andtapers forwardly between the first counterbore and the bore:(a) a rivethaving an axially extending shank receivable in the bore and defining anaxis; (b) the rivet having a head including an endface, a first sectionlocated forwardly of the endface, and a second and forwardly taperedsection located forwardly of the first section; (c) the second andforwardly tapered section of the rivet substantially conforming to saidsecond counterbore in the workpiece; (d) the first section of the rivethaving a periphery with an undeformed length which is about the same asbut not less than the length of the first counterbore in the workpiece;(e) the first section of the rivet having an undeformed outer surface ofa diameter which is less than but about the same as the diameter of thefirst counterbore so that the first section is radially outwardlydeformable into tight peripheral engagement with the first counterbore;(f) said head endface having a raised central section having asubstantially flat surface perpendicular to the axis of the rivet, theratio between the area of said raised central section and the crosssectional area of said shank being constant for all rivet sizes andtypes, the ratio being approximately 70%, said head endface taperingfrom said raised central section to the periphery of said first section.5. The combination of claim 4 wherein said first section has a peripherywith an undeformed length ranging from the same as to 0.004 inches morethan the length of the first counterbore in the workpiece.
 6. The rivetof claims 4 or 5 wherein the taper from said raised central section tothe periphery of said first section approximates a shape defined by aportion of a torus which is tangent at the periphery of said firstsection to a plane perpendicular to the rivet axis and which curves awaytherefrom to intercept said substantially flat surface of said raisedcentral section at the periphery thereof.
 7. In combination with aworkpiece having a bore, a first counterbore of a first length which issubstantially cylindrical, and a second counterbore which isfrustro-conical and tapers forwardly between the first counterbore andthe bore, the length of said first counterbore having a minimum and amaximum tolerance range;in the undeformed state: (a) the rivet having anaxially extending shank receivable in the bore and defining an axis; (b)the rivet having a head including an endface, a first section locatedforwardly of the endface, and a second and forwardly tapered sectionlocated forwardly of the first section; (c) the second and forwardlytapered section of the rivet substantially conforming to said secondcounterbore in the workpiece; (d) the first section of the rivet havinga periphery with an undeformed length which is not less than the maximumtolerance length of the first counterbore in the workpiece; (e) thefirst section of the rivet having an undeformed outer surface of adiameter which is less than but about the same as the diameter of thefirst counterbore so that the first section is radially outwardlydeformable into tight peripheral engagement with the first counterbore;(f) said head endface of the rivet having a raised central sectionhaving a substantially flat surface perpendicular to the axis of therivet, the ratio between the area of said raised central section and thecross sectional area of said shank being constant for all rivet sizesand types, the ratio being approximately 70%, said head endface taperingfrom said raised central section to the periphery of said first section;said rivet head being seated against the workpiece second counterboreand the rivet set, whereby if the length of the first counterbore is nomore than the maximum tolerance, the periphery of the first section ofthe rivet head at the endface thereof will be flush or above but notwithin the first counterbore and first section of the head of the rivetradially expanded to tightly fit within the first counterbore, and ifthe length of the first counterbore is more than the maximum tolerancelength of the first section of the rivet head, the periphery of thefirst section of the rivet head at the endface thereof will be disposedwithin the first counterbore.
 8. The combination of claim 7 whereinbefore setting, said first section has a periphery with an undeformedlength ranging from the same as to 0.004 inches more than the length ofthe first counterbore in the workpiece.
 9. The rivet of claims 7 or 8wherein before setting, the taper from said raised central section tothe periphery of said first section approximates a shape defined by aportion of a torus which is tangent at the periphery of said firstsection to a plane perpendicular to the rivet axis and which curvesoutward to intercept said substantially flat surface of said raisedcentral section.